1. Field of the Invention
The present invention relates to a hybrid stepping motor using a permanent magnet for a rotor.
2. Description of the Related Art
Recently, downsizing and power saving of motors have been highly demanded. In motors having the same external size, if a greater torque can be obtained, an electric current supplied to the motors can be reduced, thereby realizing power saving.
The technical history of stepping motors will be described while taking a 1.8 degrees step (200 steps) hybrid stepping motor as an example.
In early 1.8 degrees (200 step) hybrid stepping motor, both of a stator tooth pitch and a rotor tooth pitch are 7.2 degrees. However, in recent hybrid stepping motors, the stator tooth pitch and the rotor tooth pitch are different from each other to reduce vibrations and noises. Specifically, in a recent hybrid stepping motor, the rotor tooth pitch remains 7.2 degrees, but the stator toot pitch is usually 6.6 to 7.0 degrees.
A typical stator has six small teeth in one pole (salient pole). If the stator tooth pitch and the rotor tooth pitch are different from each other, positional relations between six stator small teeth and rotor small teeth are also different from each other. Therefore, in the recent hybrid stepping motor, a facing area of attraction poles (N pole and S pole) would be decreased, whereas a facing area of repulsion poles (N pole and N pole, S pole and S pole) would be increased compared to the early stepping motor, and therefore, a torque would be reduced. Herein, the facing area of attraction poles (N pole and S pole) and the facing area of repulsion poles (N pole and N pole, S pole and S pole) refer to those areas when the motor is in a stopped state after excitation.
In order to increase a torque, the following related-art technique can be used.
JP-A-2002-503078 discloses a hybrid stepping motor where torque per unit volume, i.e., “torque density” is optimized. According to JP-A-2002-503078, the torque density is maximized at an id/od ratio of 0.61. Also, a tooth thickness/a tooth pitch is preferably 0.38 to 0.45, and particularly 0.42.
JP-A-H5-168214 discloses a hybrid stepping motor where a ratio of an inner diameter of a stator to an outer diameter thereof is 0.62 to 0.64 in a case of two-phases and 0.605 to 0.625 in a case of five-phases in order to generate a maximum torque in a rotor. Also, it is described that the ratio of the inner diameter of the stator to the outer diameter is around 0.53 in the conventional stepping motor.
JP-A-2007-318974 discloses a brushless motor where a tooth width of a stator core is 0.14 or more times of an outer diameter of a rotor.
Regarding a torque of stepping motors, static characteristics and dynamic characteristics have factors contrary to each other, and therefore, their parameter cannot be defined in one way. However, any of the above-described motors cannot increase pull-out torque satisfactorily.